Water used in industrial cooling or mining systems comes from rivers, lakes, ponds or from underground reservoirs. Such water contains dissolved inorganic salts. When this water circulates through the heat exchangers and cooling towers in a cooling system, a portion of the water is lost due to the evaporation. This increases the concentration of inorganic salts in the system. If the solubility of these salts in water is exceeded, precipitation will take place.
As the salts precipitate on the internal surface of a cooling system, they form scale or deposits. The scale inhibits effective heat transfer, restricts the flow of the water, and promotes the development of underdeposit corrosion. Consequently, it is necessary to remove the scale by cleaning. Such cleaning is expensive because equipment must be shutdown, labor costs are incurred, and production is delayed. In view of these problems, preventing scale formation is preferred to scale removal.
Scale formation can be inhibited by adding a sequestering or chelating compound to the water treatment system. The amount of a chelating/sequestering compound required is a stoichiometric amount based upon the amount of calcium and magnesium cations in the aqueous system cleaned. This method of the scale inhibition is expensive and not customarily used.
More than 50 years ago it was discovered that certain compounds performed as highly efficient scale inhibitors. Such compounds are used in significantly lower than stoichiometric amounts and are known as "threshold inhibitors". Examples of threshold inhibitors are phosphonates and water soluble acrylic/maleic/sulfonic polymers or copolymers. Corrosion inhibitors, such as phosphonates, inorganic phosphates, azoles, zinc, and molybdate, are often used with scale inhibitors.
In addition to effective performance, water treatment chemicals must be environmentally acceptable. Environmental regulations prohibit the use of such corrosion inhibitors as chromates and restrictions are now prevalent for the use of all heavy metals. The trend is also toward water treatment chemicals that are non-toxic, have little or no phosphorus, have high calcium/hardness tolerance, are chlorine/bromine stable, and at the same time have high scale and corrosion efficacy. Because of these requirements, the cost of water has increased, causing higher reuse/higher cooling cycles which results in cooling waters with high hardness and alkalinity contents.
U.S. Pat. No. 5,523,023 relates to compositions comprising polyaspartic acid and phosphonobutane tricarboxylic acid which are used for alkaline cleaners. U.S. Pat. No. 5,386,038 discloses a water soluble mixture of phosphonated oligomers having the general formula: EQU H[CHRCHR].sub.n --PO.sub.3 M.sub.2
wherein at least one R group in each unit is a COOM, CH.sub.2 OH, phosphono sulphono, sulphato, or phosphono group and the other R group which may be the same as, or different from, the first R group, is hydrogen or a COOM, hydroxyl, phosphono sulphono, sulphato, C.sub.1-7 alkyl, C.sub.1-7 alkenyl group or a carboxylate, phosphono, sulphono, sulphato, and/or hydroxyl C.sub.1-7 alkyl, C.sub.1-7 alkenyl group, and each M is a cation such that the phosphonated oligomer is water soluble and n is 1 to 6, preferably&gt;1 and &lt;6. These compositions inhibit scale formation and/or the corrosion of metal exposed to aqueous systems.